The invention relates generally to digital subscriber line systems and more specifically to an arrangement for signal level adjustment of multitone signals in such systems.
In digital subscriber line (DSL) systems, analog signals are communicated between central offices and subscriber""s premises over standard telephone lines.
In e.g. asymmetric digital subscriber line (ADSL) systems, the analog signals are multitone signals comprising up to 255 tones with a separation of 4.3 kHz.
According to standards regulating ADSL systems, the transmitted maximum power spectral density (PSD) is specified as a constant value, i.e. it is the same for all tones or frequencies.
However, a transmitted signal will be damped over the telephone line before it reaches the other side. Moreover, the damping will vary with frequency such that tones at lower frequencies will be damped less than tones at higher frequencies. Also, the damping will vary with the length of the telephone line such that a longer telephone line will damp the signal more than a shorter telephone line.
Thus, even if the transmitted PSD is regulated by the standard to be constant, the PSD of the received signal at the receiving side will vary with frequency.
The bit rate of an ADSL link is the sum of the bits allocated to each individual tone divided by the frame time, which is regulated by standard to be 246.3 xcexcsec. It is permissible to allocate up to 15 bits per tone. The actual number of bits that can be used in practice on a tone is decided during a so-called xe2x80x9ctraining sequencexe2x80x9d that is started when an ADSL modem is powered up. During the training sequence, the far end side sends a predefined pattern with a constant PSD, while the receiving side calculates the signal-to-noise ratio (SNR) for each incoming tone. The number of bits that can be used on each tone, is then calculated by an algorithm that can be approximated by
SNR greater than 3N+9.6+Gm+Gc,
where N is the number of bits allocated to a tone, and Gm and Gc are constants.
This means that the higher the SNR is in the receiver, the more bits can be allocated to a tone.
The noise level in a receiver is usually approximately a white noise, i.e. a constant PSD for all frequencies, and often decided by an A/D converter in the receive path.
Since the receiver noise floor is approximately white and the received signal varies a lot with frequency, fewer bits can be allocated to tones at higher frequencies compared to tones at lower frequencies due to a lower SNR.
The object of the invention is to increase the bit rate in a multitone DSL system without changing the standard while still meeting the standard.
This is attained in accordance with the invention by means of a programmable filter in the receive path that boosts higher frequencies more than lower frequencies.
By the use of the programmable filter, it will be possible to obtain a flat PSD for the received signal during the training sequence.